1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus of the type including an intermediate image transfer body intervening between an image carrier and a sheet or recording medium as to image transfer.
2. Description of the Background Art
An image forming apparatus of the type described is implemented as, e.g., a color copier or a color laser printer in which toner images of different colors are transferred from an image carrier to an intermediate image transfer body one above the other (primary transfer) and then collectively transferred to a sheet (secondary transfer). The intermediate image transfer body is usually formed of a high-molecular material having a preselected mechanical characteristic and a preselected electrostatic characteristic. The problem with this type of image forming apparatus is that toner scatters around a toner image transferred from the image carrier to the intermediate image transfer body.
To obviate the scattering of the toner at the time of primary transfer, the intermediate image transfer body may include a high-resistance layer having a volume resistivity of 1010 xcexa9xc2x7cm or above. Such an intermediate image transfer body allows the potential of a latent image to be transferred from the image carrier thereto and held thereon together with the toner image. The transferred potential prevents the toner from scattering around the toner image transferred to the intermediate image transfer body.
However, the intermediate image transfer body with the high-resistance layer brings about the following problem. When a plurality of toner images are transferred to the same area of the intermediate image transfer body one above the other, the history of potential contrast images remain in the high-resistance layer in accordance with the presence/absence of toner on the image carrier and sheet. A potential contrast image left in the high-resistance layer is difficult to discharge and is apt to remain up to the next image forming cycle. As a result, when a highlight image or similar image with low image density (ID) is formed later, a residual image corresponding to the potential contrast image is likely appear in the low ID image.
We found by a series of researches and experiments that even when the intermediate image transfer body was discharged from the outside, a potential distribution remained in the body and caused a residual image to appear in an image later. Further, the potential distribution was apt to remain in the high-potential layer, which formed part of a laminate structure.
The residual charge in the intermediate image transfer body is difficult to remove with charging means that applies a DC voltage opposite to the conventional image transfer bias to the intermediate image transfer body. If the size of the DC voltage is increased, then the residual charge in the intermediate image transfer body may be discharged to a certain degree. However, such a DC voltage is likely to damage the surface of the intermediate image transfer body to a critical degree. While an AC voltage with a great amplitude may effectively discharge the residual charge, it increases the current to 1 mA or so, which is greater than several microamperes to several ten microamperes of the DC voltage. This is also likely to damage the surface of the intermediate image transfer body, and moreover increases the cost.
Particularly, as for the intermediate image transfer body with the high-resistance layer, the conventional discharging means described above simply causes the surface potential of the body to vary and cannot directly apply a bias to the high-resistance layer. It is therefore difficult to discharge the high-resistance layer with the conventional discharging means. Moreover, the conventional discharging means is apt to critically damage the surface layer.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-194967, 9-204107 and 11-231687.
It is a first object of the present invention is to provide an image forming apparatus capable of surely obviating, even when an intermediate image transfer body with a high-resistance layer is used, a residual image ascribable to polarization, which is left in the high-resistance layer, before primary image transfer.
It is a second object of the present invention to provide an image forming apparatus capable of surely obviating, even when an intermediate image transfer body of the kind is used, a residual image ascribable to polarization, which is left in the high-resistance layer, after an image forming operation.
An image forming apparatus of the present invention includes an image carrier, a latent image forming device for forming a latent image on the image carrier, and a developing device for developing the latent image with toner to thereby form a corresponding toner image. An intermediate image transfer body has a movable surface and includes an high-resistance layer whose volume resistivity is 1010 xcexa9xc2x7cm or above. A primary image transferring device transfers the toner image from the image carrier to the intermediate image transfer body. A secondary image transferring device transfers the toner image from the intermediate image transfer body to a recording medium. A polarization uniforming device uniforms, at the beginning of an image forming operation, polarization left in the high-resistance layer while preserving its polarity after the surface of the intermediate image transfer body has started moving, but before the toner image is transferred from the image carrier to the secondary image transfer body.